Stabilization of synthetic lubricants



oils do not function to a satisfactory degree.

. ments and automatic weapons.

Patented Feb. 19, 1963 3,078,229 STAEELZATIUN F .SYNTHETIC LUBRECANTS William L. Qox, Mount Prospect, lll., assignor to Universal Gil Products Company, Bes Pialnes, llh, a corporation of Delaware No Drawing. Filed May S, 1959, Ser. N $11,779 6 Claims. (ill. 252--40.7)

This invention relates to the stabilization of synthetic lubricants and more particularly to a novel method of preventing and/ or retarding deterioration thereof.

in recent years, stringent requirements for lubricants in certain applications have resulted in the availability of a new class of lubricants referred to in the art as synthetic lubricants. These lubricants do not necessarily replace petroleum oils in conventional usage, but are designed for special applications where the petroleum These synthetic lubricants have found particular use in wintergrade crankcase oils, turbo-engine oils, aviation instru- For example, aircraft gas turbines require oils capable of providing satisfactory lubrication at temperatures ranging as low as 65 F. and as high as 275 F. during use. Temperatures up to 500 F. are encountered for intervals of from one to two hours during shut-down. Petroleum lubricants are unsatisfactory at high altitudes or in the winter season for use in machine guns and automatic cannons which frequently could not be made to tire because of congealed lubricants. Because they are used under such stringent conditions, the synthetic lubricants may undergo undesirable deterioration including, for example, formation of deposits, discoloration, change of viscosity, etc. The present invention offers a novel method of retarding and/ or preventing the undesired deterioration.

The synthetic lubricants are of varied types including aliphatic esters, polyallzylene oxides, silicones, esters of phosphoric and silicic acids, highly fluorine-substituted hydrocarbons, etc. Gf the aliphatic esters, di-(Z-etbyl- "exyl) sebacate is being used on a comparatively large commercial scale. Other aliphatic esters include dialkyl azelates, dialkyl suberates, dialkyl pimelates, dialkyl adipates, dialkyl glutarates, etc. Specific examples of these esters include dihexyl azelate, di-(Z-ethylhexyl) azelate, di-3,5,5-trimethylhexyl glutarate, di-3,5,5-triinethylpentyl glutarate, di-(Z-ethylhexyl) pimelate, di-(Z-ethylhexyl) adipate, triamyl tricarballylate, pentaerythritol tetracaproate, dipropylene glycol di-pelargonate, 1,5-pentancdiol-di- (Z-ethylhexanonate), etc. The polyalkylene oxides include polyisopropylene oxide, polyisopropylene oxide diether, polyisopropylene oxide diester, etc. The silicones include methyl silicone, methylphenyl silicone, etc., and the silicates include for example, tetraisooctyl silicate, etc. The highly iiuorinated hydrocarbons in clude fluorinated oil, perduorohydrocarbons, etc.

The present invention also is applicable to the stabilization of reases made by compositing metallic soaps with the synthetic lubricating oils described above and are referred to herein as synthetic greases. These metal base synthetic greases may be further classified as lithium base synthetic grease, sodium base synthetic grease, calcium base synthetic grease, barium base synthetic grease, strontium base synthetic grease, aluminum base synthetic grease, etc. These greases are solid or semi-solid gels and, in general, are prepared by the addition to the synthetic lubricating oil of hydrocarbon-soluble metal soaps or salts of higher fatty acids as, for example, lithium stearate, calcium stearate, aluminum naphthenate, etc. The grease also may contain thickening agents such as silica, carbon black, polyacrylates, talc, etc.

The stabilization of synthetic lubricants presents problems dififerent from those encountered in the stabilization of petroleum lubricants. It has been found that certain inhibitors which are satisfactory in petroleum lubricants are unsatisfactory in synthetic lubricants, particularly at the high temperatures encountered with the synthetic lubricants. Furthermore, certain phenolic inhibitors effectively inhibit oxidation of di-(Z-ethylhexyl) sebacate at temperatures below about 250 F., but lose their effectiveness at temperatures of 325 F. and higher. At these high temperatures, the synthetic lubricants without inhibitor or with an ineffective inhibitor undergo a more rapid increase in acid number, viscosity change, sludge and lacquer formation, etc.

In one embodiment the present invention relates to a method of stabilizing a synthetic lubricant which comprises incorporating therein a stabilizing concentration of N,N,N',N'-tetraisopropyl-p-phenylene diamine.

In a specific embodiment, the present invention relates to a method of stabilizing di-(Z-ethylhexyl) sebacate which comprises incorporating therein from about 0.01% to about 3% by weight of N,N,N,N'-tetraisopropyl-pphenylene diamine.

In another specific embodiment the present invention relates to a method of stabilizing lithium base synthetic grease which comprises incorporating therein from about 0.01% to about 3% by weight of N,N,N',N-tetraisopropylp-phenylene diamine.

In another embodiment the present invention relates to a synthetic lubricant containing a stabilizing concentration of N,N,N',N'-tetraisopropyl-p-phenylene diamine.

N,N,N',N'-tetraisopropyl-p-phenylene diamine is beieved to be a novel composition of matter and, because of its unusual and superior properties as an additive to synthetic lubricants, compared to other tetra-substituted p-phenylene diamines, is being claimed as a new composition of matter.

As stated above and as will be shown in the examples appended to the present specifications, N,N,N',N-tetraisopropyl-p-phenylene diamine possesses unexpected and superior properties as an additive to synthetic lubricants. These unique properties of N,N,N',N-tetraisopropyl-pphenylene diamine in stabilizing synthetic lubricants provide a novel method of enhancing the synthetic lubricants for use under stringent conditions as hereinbefore set forth.

N,N,N,N'-tetraisopropyl-p-phenylene diamine may be repared in any suitable manner. In a preferred method, p-phenylene diamine or p-nitroaniline, or mixtures thereof, is subjected to reductive alkylation with acetone. The reductive alkylation is effected in the presence of hydrogen at a temperature of from about to about 300 C. and preferably from about to about 250 C., at a pressure of from about 100 to about 3000 pounds per square inch, although lower or higher temperatures and pressures may be used in some cases. The time of contact will depend upon whether batch or continuous type of operation is employed and may range from 15 minutes to 24 hours or more. Hydrogen is utilized in an amount of at least 4 mols of hydrogen per mol of pphenylene diamine, and at least 6 mols of hydrogen per mol of p-nitroaniline. Generally an excess of hydrogen employed and may range up to 20 or more mols of hydrogen per mol of p-phenylene diamine or p-nitroaniline. While it is preferred to utilize p-phenylene diamine or pnitroaniline as a reactant, it is understood that other suitable phenyl compounds containing nitrogen substitucuts in para position to each other may be employed such as, for example, dinitrobenzene, dinitrosobenzene, etc., with suitable modifications in the reaction conditions and concentration of hydrogen employed.

The reductive alkylation reaction is effected in the presence of a suitable catalyst. A particularly preferred catalyst comprises a composite of alumina and platinum,;

the platinum being in a range of from about 0.01% to about and preferably from about 0.1% to about 1% by weight of the catalyst. also may contain halogen and particularly fluorine or chlorine or mixtures thereof, the halogen being in a concentration of from about 0.1% to 5% or more of the catalyst. Other reductive alkylation catalysts include a mixture of the oxides of chromium, copper and barium, as well as those containing cobalt, nickel, palladium, molybdenum, etc.

In the reductive alkylation reaction, at least 4 mols of acetone are utilized per mol of p-phenylene diamine or pnitroaniline. It generally is preferred to utilize an excess of acetone and this may range up to 20 or more mols of acetone per mol of p-phenylene diamine or p-nitroaniline. Following the reductive alkylation, the product may be separated in any suitable manner to recover N,N,N',N'-tetraisopropyl-p-phenylene diamine and excess acetone. The excess acetone may be recycled for further use within the process.

While the reductive alkylation method of preparing N,N,N',N-tetraisopropyl-p-phenylene diamine is preferred, it is understood that any other suitable method of preparing this compound may be employed.

In general, the inhibitor will be utilized in a concentration of from about 0.0001% to about 5% by Weight of the synthetic lubricant, although in some cases higher or lower concentrations may be employed. The exact concentration to be used will depend upon the particular synthetic lubricant being treated. In most cases, concentrations of from about 0.01% to about 3% by weight generally will be employed.

It is understood that the inhibitor of the present invention may be used along with other additives incorporated in synthetic lubricants including, for example, corrosion inhibitors, metal deactivators, viscosity index improvers, detergents, etc. In some cases, it may be of advantage to prepare the present inhibitor as a mixture with one or more of the other additives and to incorporate the mixture in the synthetic lubricant. When desired, the inhibitor, either alone or in combination with other additives, may be prepared as a solution in a suitable solvent.

The following examples are introduced to illustrate further the novelty and utility of the present invention but not with the intention of unduly limiting the same.

EXAMPLE I N,N,N,N-tetraisopropyl-p-phenylene diamine was prepared by the reductive alkylation of p-phenylene diamine with acetone as follows: 162 grams (1.5 mols) of pphenylene diamine, 696 grams (l2 mols) of acetone, 150

grams of methanol as solvent and 100 grams of an alumina-platinum catalyst (0.4% by weight of platinum and 0.7% by weight of combined fluorine and chlorine) were reacted in an autoclave at 200 C. in the presence of 100 atmospheres of hydrogen for 6 hours, after which the methonal solvent was removed by flashing and an additional 8 mols of acetone were added and the mixture reacted at 200 C. for an additional 6 hours. The autoclave then was cooled and the reaction mixture distilled under vacuum to recover an overhead fraction distilling at 160 C. at 10 mm. pressure. The product was crystallized from methanol and 90 grams of a white solid product were recovered. The product comprised N,N,N,N'- tetraisopropyl-p-phenylene diamine, having a melting point of 36 C. and an equivalent weight of 138 which corresponds to the theoretical equivalent weight.

EXAMPLE II N,N,N,N tetraisopropyl p phenylene diamine pre pared as described in Example I was utilized as an inhibitor in synthetic lubricating oil. The synthetic lubricat- When desired, the catalyst;

ing oil is dioctyl sebacate, marketed under the trade name of Plexol 201. The evaluation was made in accordance with an oxygen stability test, in which a 100 cc. sample of the synthetic lubricating oil is placed in a bath maintained at 204 C. and air is blown therethrough at a rate of 5 liters of air per hour. The sample of synthetic lubrieating oil is examined periodically and the time to reach an acid number of 5 is reported. It is apparent that the longer the time required to reach an acid number of 5 the more stable is the sample of synthetic lubricating oil; in other words, it takes longer for the more stable oil to deteriorate.

When evaluated in the above manner, a control sample of the lubricating oil (not containing the inhibitor) developed an acid number of 5 in about 9 hours. On the other hand, a sample of the synthetic lubricating oil containing 0.0033 mol of N,N,N,N'-tetraisopropyl-p-phen ylene diamine per 100 cc. of synthetic lubricating oil, when evaluated in the above manner, did not develop an acid number of 5 until after about 44 hours.

It will be noted that the N,N,N ,N'et5tl3-iSOP1'OPY1-P- phenylene diamine effectively retarded deteriorationof the synthetic lubricating oil.

EXAMPLE III As hereinbefore set forth, N,N,N',N-tetraisopropyl-pphenylene diamine is extremely more effective than other tetra substituted p-phenylene diamines. This is illustrated in the data in the following table, which reports results of different samples of the synthetic lubricating oil described in Example 11 to which 0.0033 mol of additive were incorporated per 100 cc. of synthetic lubricating oil and subjected to the oxygen stability test described in Example II.

Table I Hours to Additive reach acid number of 5 None 9 N,N,N.N tetraisopropyl p plienylene 41 diamine. N,N dimethyl N,N di sec butyl p 18 diamine. N,N dimethyl N,N' dioetyl p phenylenc 19 diamine. N ,N dimethyl-N,N -dihexyl-pplienylene 16 diamine. 7

From the data in the above table, it will be noted that N,N,N',N-tetraisopropyl-p-phenylene diamine was over 230% as effective as the other tetra substituted p-phenylene diamines.

EXAMPLE IV A lithium base synthetic grease is prepared by mixing 9% of lithium stearate with of dioctyl sebacate. The mixture is heated at about 110 C. while agitating the same, and then is cooled to about 70 C., at which time 1% by weight of N,N,N',N'-tetraisopropyl-p-phenylene diamine is added. Agitation is continued and the mixture is allowed to cool to about 50 C., after which time "the grease is further cooled slowly to room temperature.

The stability of the grease is tested according to a modified Norma-Hoffman method in which a sample of the grease is placed in a bomb and oxygen is charged thereto. The bomb then is heated to C. and the time required for a drop of 5 pounds pressure is taken as the induction period. However, because the inhibitor of the present invention is very eifective in retarding deterioration, the run is stopped before a 5 pound drop in pressure is reached and the actual pressure drop at that time is reported.

When evaluated in the above manner, the lithium base synthetic grease containing N,N,N,N-tetraisopropyl-pphenylene diamine will not develop a 5 pound drop in pressure for a considerably longer period of time than 5 occurs in a sample of the grease not containing this inhibitor.

I claim as my invention:

1. A synthetic lubricant subject to deterioration selected from the group consisting of aliphatic esters, polyalkyleue oxides, silicones, esters of phosphoric and silicic acids, and fluorinated hydrocarbons, said lubricant containing, as an inhibitor against said deterioration, a stabilizing concentration of N,N,N,N'-tetraisopropyl-pphenyiene diamine.

2. Dioctyl sebacate containing from about 0.01% to about 3% by weight of N,N,N,N-tetraisopropy1-p-phenylene diamine.

3. A lubricant composition as defined in claim 1 further characterized in that said lubricant contains a metal soap of a higher fatty acid in sufiicient amount to form a grease.

4. The lubricant composition of claim 3 further characterized in that said soap is lithium stearate.

5. The lubricant composition of claim 3 further characterized in that said soap is calcium stearate.

6. Lithium base synthetic grease comprising a mixture of dioctyl sebacate and lithium stearate and containing from about 0.01% to about 3% by weight of N,N,N,N'-tetraisopropyl-p-phenylene diamine.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A SYNTHETIC LUBRICANT SUBJECT TO DETERIORATION SELECTED FROM THE GROUP CONSISTING OF ALIPHATIC ESTERS, POLYALKYLENE OXIDES, SILICONES, ESTERS OF PHOSPHORIC AND SILICIC ACIDS, AND FLUORINATED HYDROCARBONS, SAID LUBRICANT CONTANINGM AS AN INHIBITOR AGAINST SAID DETERIORATION, A STABLIZATING CONCENTRATION OF N,N,N'',N''-TETRAISOPROPOPLY-PPHENYLENE DIAMINE. 